1. Field of the Invention
This invention relates to shell and tube type heat exchangers, and more particularly to novel floating head means for balancing fluid forces therein, and to a novel tube support system therefor.
2. Description of the Prior Art
Floating tube sheets are known in the heat exchanger art. In these heat exchangers one of the tube sheets within which the tubes are held is allowed to move axially relative to the shell to accommodate differential axial expansion between shell and tubes. Although floating tube sheets of the prior art eliminate stresses resulting from unequal thermal expansion, they do not balance the forces acting upon the tubes due to the various fluid pressures within the heat exchanger. Therefore, even in floating tube sheet designs of the prior art, stresses still exist because of these unbalanced fluid forces.
To overcome the afore-noted drawback, the prior art teaches the use of relatively thick tubes, which result in higher material costs and have relatively poor heat transfer qualities for the fluids flowing about the tube walls. Additionally, the prior art accepts the danger of failure of at least a percentage of the tubes within the heat exchanger due to stress-produced cracking.
Another disadvantage prevalent in conventional heat exchangers is the fact that they use plates containing a large number of apertures for supporting the tubes at spaced locations along their lengths. However, use of such support plates results in an increase in abrasive wear and tear on the tubes. This abrasive wear is occasioned by the difficulty encountered in obtaining a tight and relatively slippage-free contact between the tubes and support plates.
Furthermore, the support plates direct the heat exchange medium to flow across the tube bundle or bank as opposed to an axial flow path, thereby making true counterflow heat exchange impossible, as well as increasing the pressure drop across the heat exchanger and causing acceleration and deceleration power losses along the length of the heat exchanger. Such increased pressure drop can result in an important power loss through the requirement of increased pumping power where the heat exchangers are employed in a geothermal power plant, particularly where the plant is of the type disclosed in the B. C. McCabe U.S. Pat. No. 3,757,516 which employs down-hole pumping in the geothermal well or wells to restrain geothermal hot water from flashing into steam so as to transfer heat energy at the highest possible temperature in the heat exchangers and to minimize mineral precipitation from the geothermal hot water. Shell side cross-flow of fluid in prior art heat exchanger designs also causes reduced fluid velocities, which, among other things, results in lower heat transfer coefficients.
The various inefficiencies, including those noted above, in prior art cross-flow heat exchanger designs generally require on the order of about twice or more fluid contacting surface than would be required for a true conterflow configuration, resulting in undesirably heavy construction and attendant difficulties in handling and mounting.
Reduced fluid velocities in cross-flow heat exchangers not only results in lower heat transfer coefficients as noted above, but also increases the chances of silting and fouling, and increases the likelihood of corrosion of some tube materials which require high surface velocities to prevent corrosion. Where solids may be entrained in the shell side fluid, prior art cross-flow increases the chances for impingement pitting, and in general increases the erosion effect on the tubes and tube supports in the heat exchanger. Corrosion, fouling and coping with entrained solids are all serious problems in geothermal plant heat exchangers, particularly where the geothermal energy resource is a very hot brine of high chloride and silica content such as is found in California's Imperial Valley.
There are several additional generally structural problems associated with the prior art use of apertured support plates for supporting the tube bundle at intervals along the shell. One very basic problem is that such construction makes assembly of the tube bundle a very difficult, time-consuming, and expensive proposition. Also, the spaced tube support plates have relatively large surface areas of engagement with the outer shell, and thereby do not readily slide relative to the shell, so that they tend to resist adjustment for unequal thermal expansion and thereby introduce undesired stresses throughout the heat exchanger. Further, prior art tube bundle supports generally necessitate a considerable amount of peripheral spacing between the outermost tubes of the bundle and the outer shell, whereby a substantial amount of fluid tends to bypass the desired heat exchange contact with the tubes, and the fluid flow rate correspondingly decreases and pressure drop increases in the region of the interstices between the tubes of the bundle.
The failure of prior art heat exchangers to balance the forces acting upon the tubes due to the various fluid pressures within the heat exchanger, and the generally insecure prior art means for tying and supporting the tube bundle, present particularly critical problems where the heat exchanger is employed in a geothermal energy system of the type disclosed in said McCabe U.S. Pat. No. 3,757,516 where geothermal hot water is normally prevented by pump means from flashing into steam. Should such pump means inadvertently fail, the resultant pressure drop would allow some of the geothermal hot water to flash into steam, and that could collapse again, resulting in severe stressing and possibly even an explosive situation in connection with such prior art heat exchanger.
Further difficulties in the prior art are also encountered in attempting to prevent corrosion in the typical heat exchange components, as well as in providing adequate lubrication for the movable parts therein.